1. Field of the Invention
This invention relates to detection of explosives and more particularly to an ion source of an ion mobility spectrometry instrument that detects chemicals present as vapors in air or other gases, or liberated as vapors from condensed phases such as particles or solutions.
2. Description of Related Art
Ion mobility spectrometer (IMS) instruments operate on the basis of the time taken by ionized molecules to move through a gas-filled drift region to a current collector while under the influence of an electric field. The ions are typically created in an atmospheric pressure gas-filled chamber called the ion source, which is connected to a drift chamber through an orifice or a barrier grid. The ion source may use any of a variety of techniques to ionize atoms and molecules. One or more flowing streams of gas enter the ion source through one or more orifices, and the gas may exit through one or more different orifices. At least one of the flowing gas streams entering the ion source typically includes gas that has been sampled (the “sample gas”) from the surrounding atmosphere or other source of vapor to be analyzed.
In some cases, the process of taking a sample begins with an operator rubbing an absorbent substance, such as chemical filter paper, onto the surface to be tested. Particles of the chemical of interest may then be transferred and concentrated on the absorber. This intermediate absorber is then brought to the vicinity of the sampling orifice of the IMS. The quantity of particles of the target substance on the target surface is usually very small, often corresponding to only nanograms or even picograms of particles per square centimeter. The IMS must be very sensitive to identify a valid signal from evaporated target molecules when the initial concentration and surface area of target particles is so small.
A sampling method that is employed is to provide a gas pump, which draws the sample gas into the ion source through a tube. For example, the pump may be disposed to provide a partial vacuum at the exit of the ion source. The partial vacuum is transmitted through the confines of the ion source and appears at the entrance orifice of the ion source. A further tubulation may be provided as an extension to a more conveniently disposed sampling orifice external to the IMS. The operator places a sample in the near vicinity of this external sampling orifice, and the ambient vapor is drawn into the gas flow moving towards the ion source.
The ion source of the IMS provides a cloud of charged particles that is approximately proportional to the concentration of target molecule vapor, as well as some other background molecules. This concentration is further dependent on the equilibrium vapor pressure of the target molecule, the temperature of the target molecule where it is emitting the vapor, the total flow rate of non-target gas that dilutes the target vapor, and possible adsorption losses on surfaces of the gas sampling system. Existing systems that utilize absorbent surface concentration sometimes employ an oven to greatly warm the absorbent material, often up to 200°, and thereby increase the target vapor concentration.
In some circumstances, it is desirable for IMS instruments to be able to sample vapors at a distance from the external sampling orifice. Examples may include, but not be limited to, sampling of vapor from complex surfaces that contain many holes, crevices, or deep depressions, textured materials such as cloth, people and animals that prefer not to be rubbed by absorbent material, large three dimensional objects, surfaces that must be sampled in a short time, and surfaces in which surface rubbing by human operators is inconvenient or expensive. In addition, it has been observed that the sampling orifice may become contaminated with vapor-emitting particles if the sample inadvertently contacts the orifice. Such contamination is particularly difficult to remove in a short period of time, thus preventing continuous operation of the instrument. Such contamination could be avoided if vapors are sampled at a distance from the sampling orifice.
The ion source found in the IMS is typically a radioactive beta source of nickel-63. The high energy beta particles have sufficient energy to ionize the surrounding gases. Other types of ion sources that have been reported include corona discharge, electric discharge, laser, x-ray, photoelectric effect, and short wavelength ultraviolet. The ion sources fall into two categories, those that produce relatively high energy electrons and those that produce relatively low energy electrons. The photoelectric effect and laser ion sources are in the latter category and thus are not as prone to decomposing delicate organic molecules.
The photoelectric effect is the emission of an electron from a surface that is induced by the absorption of a high energy photon, typically in the ultraviolet portion of the electromagnetic spectrum. This surface is called a photocathode. Typically, a solid electrically conducting surface, a semi-transparent electrically conducting surface, or an electrically conducting mesh is employed as the electron emitting surface. Another example that appears superficially similar to the corona source is a photocathode shaped like a needle or wire. In this case the photocathode is biased to a voltage that is not sufficient to induce significant corona emission but is dependent on photon absorption to emit electrons. The requirement for electrical conductivity is due to the surface charging that would occur for insulating surfaces. After an initial emission of electrons, subsequently emitted electrons would be attracted back to the original emitting surface due to the build-up of positive surface charge. Additionally, the electrically conducting surface must be in electrical communication with the destination for the emitted electrons in order to complete the circuit. Additionally, an ambient electric field is required to draw the emitted electrons away from the emitting surface, since their initial energy is very small, typically much less than 3 electron Volts, and they can readily be attracted back by even a small unbalanced residual positive charge on the emitting surface.
The electrons produced by the ion source readily combine with oxygen and other naturally occurring gases in the atmosphere. Alternatively, a special “reactant” gas species may be combined with the inflowing sample gas. The reactant gas may be selected for specific chemical and electron affinity properties in order to make the ion mobility spectrometer more sensitive or selective to specific chemical species. The electrically charged molecules, whether a naturally occurring gas or a special reactant gas, then transfer their charge by a variety of reactions to the target molecule of interest.
It is well known that a photocathode is sensitive to various forms of surface contamination, such as oxidation or coating. This can be particularly severe for an IMS, where the photocathode is operated at atmospheric pressure, rather than a vacuum. The gradual build up of contamination causes a gradual decrease in the emission of electrons, often by creating a barrier to passage of the lowest energy electrons in the emission spectrum or by creating a semi-insulating surface layer that retains positive charge long enough to inhibit electron emission.
A photoelectric effect ion source may be operated in continuous or pulsed modes. In pulsed mode an electric arc discharge flash lamp, such as a xenon flash lamp, may be employed as the source of ultraviolet (UV) photons. In continuous mode a short wavelength krypton, xenon, mercury, or deuterium arc lamp may be employed.
The ambient electric field required to draw the electrons from the photocathode is required only while the source of photons is in operation. After a sufficient delay to allow adequate separation of the electrons from the photocathode surface, the ambient electric field may be increased, decreased, or turned off as required for the desired operation of the ion mobility spectrometer.